1. Field of the Invention
The present invention generally relates to a method for checking (or verifying) the composition of multilayer units composed of sheetlike parts. The units are passed in interspaced succession by a checkpoint where a value varying with the thickness of the unit is determined. This value is compared with a reference value corresponding to the desired thickness of the unit. The unit is then accepted or rejected depending on the results of the comparison. The invention also relates to an apparatus for composing and checking the composition of such units.
2. State of the Art
Bulk mail distributions often require the automatic preparation of multilayer units that are composed of sheetlike parts. These units are typically checked to make sure that they contain the correct number of sheets before they are mailed. Conventional checking techniques use a microswitch which is manually set to the desired thickness for each unit. The units are then checked for that one desired thickness as they pass a checkpoint. Conventional checking methods may, therefore, help prevent a situation where one document clings to another document (as a result of static electricity, for example) and is mailed to the wrong person as part of the wrong unit. However, the incorrect composition of a unit may also be due to other factors such as a feed station that fails to supply the correct number of documents for any unit.
One limitation of such conventional checking techniques is that the reference value (and hence the microswitch) must be manually reset for the desired composition (or thickness) of each different type of unit. Since it becomes increasingly difficult to accurately detect discrepancies in the overall composition of a unit as it includes more parts, eventually a point is reached where manually resetting the microswitch becomes too difficult to be entrusted to unskilled personnel.
Furthermore, since microswitches change signals at only one given value, they are not suitable for mail processing systems that form consecutive units having different compositions. For example, conventional microswitch detection systems may be unsuitable for bulk mail processing systems using optically read indicators that allow different documents to be selectively added to consecutive envelopes.
German Patent Application 32 21 379 discloses a method and apparatus for detecting whether 0, 1, or more sheets pass a checkpoint. The apparatus in that publication is intended to ascertain whether a single sheet--no more and no less--is fed to a paper printing apparatus. The device uses electromagnetic coils to measure the displacement of a roller from a support. The roller is pushed up by the sheetlike parts as they pass between the support and the roller.
There are various reasons why devices such as the one described in German Application 32 21 379 are unsuitable for use with composite units having multiple sheetlike parts. For example, a composite unit (and in particular a unit that includes folded documents) may be thick enough to cause the roller to jump, or bounce, to a level above the upper surface of the unit upon initial contact with the edge of that unit. This bouncing can give rise to incorrect or inaccurate measuring results. Similar inaccuracies might also be caused by an uneven arrangement of the various component documents in a unit. Furthermore, some units may not have a consistent thickness; especially if the unit includes documents having different lengths or documents which are folded into unequal portions.
Conventional detection devices are usually limited to merely determining whether a document has, in fact, been added to a unit. Therefore, conventional techniques usually require a detection apparatus at each document feed station. Moreover, such conventional techniques require each composite unit to be periodically checked at various stages in the assembly process in order to verify that every document supplied to that unit remains with the unit.